BNB Smart Chain Migration Guide

Across supports bridging USDC, USDT, and ETH to and from BNB Smart Chain (Chain ID 56). This guide covers the API changes required for developers, integrators, and relayers.

BNB Smart Chain is the first chain using Across V4's ZK settlement. No testnet is available — test on mainnet with small amounts.

API Changes

New `outputAmount` Field

The /suggested-fees endpoint now returns a calculated outputAmount field directly. Stop calculating output amounts manually.

output-amount-change.ts

// Old approach (deprecated) — do NOT use
const outputAmount = inputAmount * (1n * 10n**18n - BigInt(fees.totalRelayFee.pct)) / 10n**18n;

// New approach (required) — use the API response directly
const outputAmount = fees.outputAmount;

`allowUnmatchedDecimals` Parameter

A new optional query parameter allowUnmatchedDecimals has been added to /suggested-fees. When omitted, the API returns a 4XX error if input and output tokens have different decimal counts (e.g., USDC has 6 decimals on some chains, 18 on others).

Set allowUnmatchedDecimals=true if you're intentionally bridging between tokens with different decimals.

`outputToken` Requirements

For BSC routes, you must NOT set outputToken to 0x0. Integrators must explicitly specify the output token address for all BNB Smart Chain transfers.

Deprecation Timeline

Date	Change
Now	`outputToken=0x0` still works on non-BSC routes
June 19, 2025	Protocol contracts will prevent `outputToken=0x0` network-wide

Action required: Update all integrations to explicitly specify outputToken addresses before the June deadline.

For Relayers and Indexers

Recognize chain ID 56 in your configurations
Reject any BSC deposits where outputToken is set to 0x0
Prepare for complete outputToken=0x0 removal by June 19, 2025

Key Details

Property	Value
Chain ID	`56`
Supported tokens	USDC, USDT, ETH
Deposit function	`depositV3()`
Settlement	Across V4 (ZK proofs via Succinct SP1)
Testnet	Not available
App-SDK	Supported

BNB Smart Chain Migration Guide

API Changes

New outputAmount Field

allowUnmatchedDecimals Parameter